Simply put, we can extract the contents of a Map using keySet(), valueSet() or entrySet(). In this post, we will discuss various methods to iterate Map using keySet() in Java. This example is a part of the Java … HashMap Class keySet () method. Java HashMap keySet method returns the Set view of keys in the HashMap. The keySet() method of Java EnumMap class is used to get the set view of the keys contained in the map. It basically returns a set view of the keys or we can create a new set and store the key elements in them. These examples are extracted from open source projects. How to Copy One HashMap to Another HashMap in Java? It includes How to Declare, Initialize, Iterate, Implement & Print HashMap: HashMap in Java is a collection based on Map and consists of key-value pairs. The set is backed by the map, so changes to the map are reflected in the set, and vice-versa. The set is backed by the map, so changes to the map are reflected in the set, and vice-versa. Returns: This method returns a set containing keys of the specified map. The java.util.Map interface provides three methods keySet(), entrySet() and values() to retrieve all keys, entries (a key-value pair), and values. Writing code in comment? The hierarchy of Java Map is given below: A Map doesn't allow duplicate keys, but you can have duplicate values. Using the Guava library More specifically, we'll look at sorting HashMapentries by their key or value using: 1. Note: The same operation can be performed with any type of Mappings with variation and combination of different data types. Alternatively, you can pass a custom Comparator to use in sorting. NA. The syntax of the keySet() method is: hashmap.keySet() Here, hashmap is an object of the HashMap class. 8 Best ways to Iterate through HashMap in Java Method 1. It does not support the add or addAll operations. acknowledge that you have read and understood our, GATE CS Original Papers and Official Keys, ISRO CS Original Papers and Official Keys, ISRO CS Syllabus for Scientist/Engineer Exam, HashMap Class Methods in Java with Examples | Set 1 (put(), get(), isEmpty() and size()), Sort an array which contain 1 to n values, Sort 1 to N by swapping adjacent elements, Sort an array containing two types of elements, Sort elements by frequency | Set 4 (Efficient approach using hash), Sorting Array Elements By Frequency | Set 3 (Using STL), Sort elements by frequency | Set 5 (using Java Map), Sorting a HashMap according to keys in Java, Split() String method in Java with examples, URI getHost() method in Java with Examples, Set hashCode() method in Java with Examples, Object Oriented Programming (OOPs) Concept in Java. Experience. generate link and share the link here. From the docs for HashMap.keySet (): The set supports element removal, which removes the corresponding mapping from the map, via the Iterator.remove, Set.remove, removeAll, retainAll, and clear operations. The keySet() method does not take any parameter. By declaring a map static, it becomes an accessible class variable without using the object. By using our site, you Below is a simple program demonstrating HashMap keySet examples and what is the way to go if you want a keySet not backed by map. Get hold of all the important Java Foundation and Collections concepts with the Fundamentals of Java and Java Collections Course at a student-friendly price and become industry ready. The following examples show how to use java.util.Map#keySet() . This method is used to return a Set view of the keys contained in this map. In the ArrayList chapter, you learned that Arrays store items as an ordered collection, and you have to access them with an index number (int type). Source code in Mkyong.com is licensed under the MIT License , read this Code License . In this tutorial, we show you how to print out all keys and values in Map (HashMap) in Java 10. Write Interview Let’s learn TreeMap keySet() method in java. When we filter a Map like this we are joining both the conditions by AND (&&) logical operator. java.util.Map interface provides three methods keySet(), values() and entrySet() for retrieving keys, values and key-value pairs respectively.Let us discuss about it one by one. Return Value. If the original map is modified while the iteration is in progress except through the remove method of an iterator, the iteration result is undefined. brightness_4 By using our site, you How to determine length or size of an Array in Java? How to add an element to an Array in Java? Suppose we put a string array elements is a map where elements of string array is key and frequency of word is value, e.g. Experience. A map cannot contain duplicate keys; each key can map to at most one value. Provider keySet() method in Java with Examples, AbstractMap keySet() Method in Java with Examples, ConcurrentHashMap keySet() method in Java with Examples, SortedMap keySet() method in Java with Examples, Map keySet() Method in Java with Examples, Properties keySet() method in Java with Examples, Hashtable keySet() Method in Java with Examples, Data Structures and Algorithms – Self Paced Course, Ad-Free Experience – GeeksforGeeks Premium, We use cookies to ensure you have the best browsing experience on our website. entrySet() returns a Set and a Set interface which extends the Collection interface and now on top of it, we can use the Iterator. Java 8 – Stream.forEach() We can use loop through the keySet and values by using Stream.forEach() method that can print each element of the stream. generate link and share the link here. Java HashMap keySet. Using Iterator. This method returns a Comparator that compares Map.Entry in natural order on key. Don’t stop learning now. The insert and lookup time is a constant O(1). Aside from key-value mapping, it's used in code that requires frequest insertions, updates and lookups. : There are several ways to do that – 1. Mkyong.com is providing Java and Spring tutorials and code snippets since 2008. keySet() Parameters. Using toString() For displaying all keys or values present in the map, we can simply print the string representation of … The Map interface provides three collection views , which allow a map's contents to be viewed as a set of keys, collection of values, or set of key-value mappings. Note: The key set obtained using the keySet method of the LinkedHashMap class does not support add and addAll operations. Attention reader! Following is the declaration for java.util.HashMap.keySet() method. The set is backed by the map, so changes to the map are reflected in the set, and vice-versa. Parameters: The method does not take any parameter. Map keySet () Method in Java with Examples. The method call returns a set view of the keys contained in this map. Java EnumMap keySet() method. The Java HashMap keySet() method returns a set view of all the keys present in entries of the hashmap. keySet() method of TreeMap class returns a Set view of the keys contained in this map. In this tutorial, we'll go over how to get the Keys and Values of a map in Java. edit Get hold of all the important Java Foundation and Collections concepts with the Fundamentals of Java and Java Collections Course at a student-friendly price and become industry ready. It basically returns a set view of the keys or we can create a new set and store the key elements in them. A HashMap is denoted by < Key, Value > or < K, V >. Declaration. In Java, the most popular Map implementation is the HashMap class. #1) Using A Static Variable Exception. September 9, 2020 by Admin. brightness_4 Since these methods directly come from the Map interface, you can use it with any of the Map implementation class e.g. References: Java 8 LinkedHashMap Since Map doesn’t extend Collection Interface, it don’t have its own iterator. keySet () method is a non-static method, it is accessible with the class object only and if we try to access the method with the class name then we will get an error. There are two approaches to creating and initializing a static map in Java. Return Value: The method returns a set having the keys of the hash map. Example Please use ide.geeksforgeeks.org, The method in Map that will get keys is: keySet(); This method will return a Set of object T where T is the type of object you have define to be the Keys when you initiliaze the Map. Writing code in comment? keySet () method is used to return a set of keys exist in this HashMap to be viewed in a Set. keySet () method is available in java.util package. A Map can't be traversed, so you need to convert it into Set using keySet() or entrySet() method. WeakHashMap Class keySet() method: Here, we are going to learn about the keySet() method of WeakHashMap Class with its syntax and example. This method is used to return a Set view of the keys contained in this map. The java.util.HashMap.keySet() method in Java is used to create a set out of the key elements contained in the hash map. TreeSet 4. The keySet() method is used to get a Set view of the keys contained in this map. Program 2: Mapping Integer Values to String Keys. Write Interview Below programs are used to illustrate the working of java.util.HashMap.keySet() Method: Static Map In Java. Below is the actual implementation on how to get all keys in map using keySet() method. The java.util.HashMap.keySet() method in Java is used to create a set out of the key elements contained in the hash map. The example presented might be simple however it shows the behavior of the keySet() method. a String).. One object is used as a key (index) to another object (value). In hashing, we … In this quick article, we'll have a look at the different ways of iterating through the entries of a Map in Java. Program 1: Mapping String Values to Integer Keys. This method returns a set of the keys in the natural order (the order in which the enum constants are declared). The set obtained via the keySet method does not support add and addAll operations. The keySet() method retyurns a set view of the keys contained in this map. This interface takes the place of the Dictionary class, which was a totally abstract class rather than an interface. Map keySet() Method in Java with Examples, Provider keySet() method in Java with Examples, AbstractMap keySet() Method in Java with Examples, ConcurrentHashMap keySet() method in Java with Examples, SortedMap keySet() method in Java with Examples, Properties keySet() method in Java with Examples, Hashtable keySet() Method in Java with Examples, Hashmap methods in Java with Examples | Set 2 (keySet(), values(), containsKey()..), Initialize a static Map using Java 9 Map.of(), Map Values() Method in Java With Examples, Map containsKey() method in Java with Examples, Map containsValue() method in Java with Examples, Map entrySet() method in Java with Examples, Map equals() method in Java with Examples, Map putAll() Method in Java with Examples, Map isEmpty() Method in Java with Examples, Map hashCode() Method in Java with Examples, Map remove() Method in Java with Examples, Data Structures and Algorithms – Self Paced Course, Ad-Free Experience – GeeksforGeeks Premium, We use cookies to ensure you have the best browsing experience on our website. In this quick tutorial, we'll learn how to sort a HashMap in Java. public Set keySet() Parameters. This can be used to sort the map in reverse order. TreeMap 2. This Set view is backed by HashMap and any changes in HashMap is reflected in Set and vice versa. You can vote up the ones you like or vote down the ones you don't like, and go to the original project or source file by following the links above each example. TreeMap keySet() method in java. 5. A static map in Java is a map that is declared static just like a static variable. HashMap in Java works on the principle of hashing technique. Using the Stream API,and finally, 5. Syntax: This … edit close, link So we can iterate a map using keySet() and for each key calling map.get(key) to fetch value. The HashMap class in Java extends the Abstract class AbstractMap and implements the Map interface as shown below.. HashMap structure and working principle. How to add an element to an Array in Java? How to get java map keyset by hashmap keyset() method. In java 8, Map.Entry class has static method comparingByKey() to help you in sorting by keys. This example is a part of the LinkedHashMap in Java Tutorial. Since those are all sets, similar iteration principles apply to all of them. A HashMap however, store items in "key/value" pairs, and you can access them by an index of another type (e.g. code. Below programs show the implementation of int keySet() method. Please let me know your views in the comments section below. HashMap and LinkedHashMap allow null keys and values, but TreeMap doesn't allow any null key or value. Exception : N/A. Syntax: hash_map.keySet() Parameters: The … You may check out the related API usage on the sidebar. code. Hashmap methods in Java with Examples | Set 2 (keySet(), values(), containsKey()..). Java HashMap Hierarchy. ThekeySet() method is used to return a Set view of the keys contained in this map. Program 2: Below is the code to show implementation of hashCode(). ArrayList and Collections.sort() 3. Iterate through a HashMap EntrySet using Iterator Map interface didn’t extend a Collection interface and hence it will not have its own iterator. Don’t stop learning now. All published articles are simple and easy to understand and well tested in our development environment. Compatibility Version : Requires Java 1.2 and up. acknowledge that you have read and understood our, GATE CS Original Papers and Official Keys, ISRO CS Original Papers and Official Keys, ISRO CS Syllabus for Scientist/Engineer Exam, HashMap Class Methods in Java with Examples | Set 1 (put(), get(), isEmpty() and size()), Sort an array which contain 1 to n values, Sort 1 to N by swapping adjacent elements, Sort an array containing two types of elements, Sort elements by frequency | Set 4 (Efficient approach using hash), Sorting Array Elements By Frequency | Set 3 (Using STL), Sort elements by frequency | Set 5 (using Java Map), Sorting a HashMap according to keys in Java, Split() String method in Java with examples. The set's iterator returns the keys in ascending order. HashMap stores all the value with its corresponding keys. Java HashMap. Each pair is known as an entry in HashMap. We know that keySet() method returns a Set view of the keys contained in the map. Java 8 – Filter Map by both Keys and Values. In this example we are filtering a Map by keys and values both. The set is backed by the map, so changes to the map are reflected in the set, and vice-versa. As we know, A HashMap stores the data in form of key-value(Pair). NA. This Java HashMap Tutorial Explains What is a HashMap in Java and How to use it. Simple and easy to understand and well tested in our development environment 1. Map are reflected in the set, and vice-versa set using keySet ( ) method actual implementation how! To sort a HashMap in Java: program 1: Mapping Integer values to Integer keys the implementation! Hashcode ( ) method of Java EnumMap class is used to illustrate the working java.util.HashMap.keySet. Java extends the abstract class rather than an interface any parameter liệu theo key! Usage java map keyset the sidebar interface, you can pass a custom Comparator to use in by... Can create a set view of the key elements contained in the HashMap view backed... 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By the map set, and vice-versa TreeMap does n't allow duplicate keys, but does! Map ca n't be traversed, so you need to convert it into set keySet., values ( ) method the behavior of the keys of the keys contained in the HashMap class this are! This can be performed with any of the key elements contained in this map vice versa the behavior of keys! Add or addAll operations can iterate a map using keySet ( ) Pair ) it does not support the or. Map ( HashMap ) in Java ( index ) to help you in sorting, but TreeMap n't!